President Reagan marked George Washington's 250th birthday yesterday by placing a wreath on the first president's tomb at Mount Vernon.

In a speech from the steps of Washington's home to a small, chilled crowd of invited guests, Reagan hailed Washington for meeting the challenges of his time and pledged that "We can, we must and we will meet ours." The speech was broadcast to classrooms across the country in a joint project of the White House and Mutual Broadcasting System.

Reagan's visit to Mount Vernon closed the historic site to visitors for the entire morning. Among the disappointed people were 60 kindergarten students from Braddock Elementary School in Annandale. Pris Joiner, a parent, said that the 5- and 6-year-olds had been planning to visit Mount Vernon on George Washington's Birthday for months and it was to be their only school outing of the year.

Security officials turned them back at the gates, she said. Mount Vernon reopened to the public at 1 p.m. after Reagan had departed.

Reagan and his wife, Nancy, flew by helicopter to a field near the tomb where the wreath was placed as men in revolutionary-era blue coats stood at attention and a bugler played taps.

Reagan's speech was a recitation of high points in Washington's life. "Never a passive leader, never an armchair general, he was always in front of his troops and his nation. He did more than live up to the standards of the time. He set them," Reagan said.

"Let us ask ourselves, 'Are we keeping faith with his trust in us?' The problems we face today don't require the kind of sacrifices Washington and his men made that Christmas night 1776 on the Delaware. But they do require us to give and sustain our best efforts--to believe in each other, to believe in the God who has blessed us and will help us to rebuild our country," Reagan said.

Reagan addressed the schoolchildren listening in their classrooms: "If Washington seems much larger than life and makes you feel a little smaller, I'll let you in on a secret--he makes us all feel that way."

Lafayette once asked Washington why Americans remain confident and happy even in desperate times. " 'There is freedom. There is space for a man to be alone and think and there are friends who owe each other nothing but affection,' " Reagan quoted Washington as replying.

"We still have that in America," Reagan said. "As Americans, let us all rededicate ourselves to the ideals that George Washington set," he said.

The president answered one shouted question from a reporter as he got into his limousine to ride to Washington's tomb. He said he did not read last week's Washington Post-ABC News poll as reflecting a people's desire for a change in economic course.

"They may have some complaints about things, but they don't want the program changed," Reagan said.

Will you change your program, he was asked.

"No," he answered.